1. Field of the Invention
This invention relates to an automatic gas analysis system for measuring the contaminants in the exhaust gas from the motor vehicle, and particularly to a system using a nondispersive infrared gas analyzer in which the exhaust gas to be analyzed is passed through a sample cell only during the time that measurements of the contaminant level are made, the sample cell being purged with clean ambient air to remove the exhaust gas from the sample cell during the times when measurements are not being taken. The present invention improves the accuracy of the measurements by purging the exhaust gas transport system of any exhaust gas which may remain in the gas transport system, and prevents the pollutants in the exhaust gas from fouling the analyzer as would occur if exhaust gas remained in the system continuously.
2. Description of the Prior Art
Public concern with environmental pollution, particularly that caused by the contaminants emitted in the exhaust gas emissions from motor vehicles, has resulted in the development of exhaust gas analysis systems which measure the amount of selected contaminants in the exhaust gas emissions. Many such systems use nondispersive infrared gas analyzers in which the emissions gas from a motor vehicle is fed into a sample cell and an infrared light beam passed through the sample cell, the amount of infrared light absorbed at a particular wavelength being indicative of the amount of the selected contaminant in the exhaust gas.
It has been found that inaccuracies occur in the measurements due to the hang up of exhaust gas in the gas transmission lines and gas transport system including the pumps which feed the exhaust gas from the motor vehicle to the sample cell in the infrared gas analyzer. It has further been found that the pollutants in the exhaust gas cling to the walls of the sample cell and reduce the transmission of infrared light through the sample cell if the exhaust gas is allowed to remain in the sample cell for a long period of time. When the sample cell and other optical equipment in the infrared gas analyzer become coated with pollutants, frequent cleaning of the unit is required in order to prevent deterioration in the reliability of measurements produced by the unit.